Why would there be a runtime flag and not just the appropriate binary installed?
I think that’s quite a corner-case.
Not at all; did all the time when the PowerPC to Intel transition happened.
If you have an Intel Mac and wanted to use your universal apps on a friend’s M1 Mac mini with your operating system configured the way you like it, that's by far the easiest way to go.
I upgrade to the universal versions of Affinity Photo, Designer and Publisher earlier today; if I had to do something complex and could use an M1 Mac, that's exactly what I'd do.
Oddly, the imac is long gone, but the mini can still run ubuntu, and is still (somewhat) useful for testing bigendian bugs. It's slower than a raspberry pi of a few years back.